1. Field of the Invention
The invention relates to a process for producing 1,3-substituted tetrahydro-1H-thieno-[3,4-d]-imidazol-2(3H)-on-4-ylidene pentanoic acid ester as suitable precursors for producing (+)-biotin.
2. Background Art
(+)-Biotin is known as a human vitamin, vitamin H. But biotin is also used as a pharmaceutical active substance for the treatment of dermatosis and as a feed additive with growth enhancing effect for working animals.
From European Published Patent Application No. 273,270 it is known that, in an intermediate stage in the synthesis of (+)-biotin, 1,3-substituted tetrahydro-1H-thieno-[3,4-d]-imidazol-2(3H)-on-4-ylidene pentanoic acid ester is produced from 1,3-substituted tetrahydro-1H-thieno-[3,4-d]-imidazol-2,4-dione by a Grignard reaction or by a Wittig reaction. However, the very poor yields of 12 percent with the Wittig reaction or 28 percent with the Grignard reaction are a serious disadvantage.